1. Field of the Invention
The present invention relates to a tool and a method for the abrasive machining of a substantially planar surface.
2. The Prior Art
Tools of the generic type are, in particular, lapping wheels and grinding wheels which are required for the machining of disk-like workpieces, in particular of semiconductor wafers, and dressing rings which are used to level the working surfaces of lapping wheels and grinding wheels.
It is customary for such a tool to be made from a casting and to be prepared for use by machining and, if appropriate, local post-hardening. Particularly if the tool is required in connection with the production of semiconductor wafers, is has to satisfy stringent quality requirements. For example, fluctuations in terms of the hardness and structure of the crystal microstructure of the tool are to be kept within extremely narrow tolerance ranges. Particularly if this is a tool with a large working surface, this requirement can only be fulfilled with a considerable outlay in terms of machinery. The use of a tool with different local hardnesses and an inhomogenous microstructure is not acceptable, however, since it cannot uniformly machine workpieces.
It is an object of the present invention to provide a procedure for fulfilling the requirement in a simpler and more advantageous way.
The present invention relates to a tool for the abrasive machining of a substantially planar surface, which has a base body and a plurality of elements which are mounted on the base body. These elements form a planar coating which is used as the working coating during machining of the surface.
The present invention also relates to a method for the abrasive machining of a substantially planar surface using a tool, wherein a plurality of elements are mounted on a base body. The elements form a planar coating which is used as a working coating during machining of the surface.
The present invention has a number of particular advantages. For example, a smaller cast mass is required to cast one element and can be controlled more easily. Thus bodies with a more homogenous microstructure and hardness are obtained even during casting of the elements than would be the case in a single-piece cast body with the mass of a plurality of elements. Furthermore, a plurality of individual elements can be machined further more easily than one casting of the same mass. A particular advantage is that the individual elements can be examined in terms of their properties such as microstructure and hardness and can be selected for use in the tool. In this way, it is possible to obtain a tool whose properties, such as for example the microstructure and hardness, lie within a particularly narrow tolerance range and are accurately matched to the object which is to be achieved using the tool. An element which has not been selected, for example because its hardness was not within the required tolerance range, is stored. This storage continues until a tool is to be provided for a task whose execution requires a working coating whose properties correspond to those of this stored element in storage.
In contrast to the elements, the base body is scarcely subject to wear and can therefore be reused to a greater extent than the elements. It is preferable for a base body to be used only on a single machine. This avoids prolonged positioning times which are required during a tool change. Therefore, it is particularly preferable to provide two base bodies for one machine, which bodies are used alternately on this machine.
The elements are made from cast metal and only those elements which have a substantially uniform microstructure and hardness are mounted on the base body.
The tool according to the invention is preferably designed as a lapping wheel for lapping or a grinding wheel for grinding disk-like workpieces, such as semiconductor wafers, or as a dressing tool for the abrasive machining of such lapping and grinding wheels.